When a non-aqueous electrolyte secondary battery is overcharged, excessive lithium is released from the positive electrode, and the thermal stability of the positive electrode lowers. The lithium released from the positive electrode is storaged by the negative electrode. However, if excessive lithium is released from the positive electrode, lithium deposits on the surface of the negative electrode. In this case, the thermal stability of the negative electrode lowers significantly as the positive electrode. Eventually, the battery overheats and the safety degrades.
To address this problem, there has been proposed the addition of a small amount of biphenyl, which is an aromatic compound, to the non-aqueous electrolyte of a battery (see Patent Document 1). There has also been proposed the addition of terphenyl and terphenyl having an arkyl group to the non-aqueous electrolyte (see Patent Document 2). These compounds polymerize when the battery is overcharged. As a result, the pores of the separator become clogged, so that the internal resistance of the battery rises and the safety improves.
However, biphenyl, terphenyl, and terphenyl having an alkyl group are solid and have a low solubility in non-aqueous solvents. Thus, at low temperatures, part of these compounds deposit, thereby resulting in degradation of battery performance. Also, these compounds have low oxidation potentials, and at high temperatures, they react with a positive electrode material during storage and charge/discharge cycles, thereby resulting in degradation of battery performance.
Further, in order to ensure safety of an overcharged battery without adversely affecting low temperature characteristics and storage characteristics, there has also been proposed the addition of a small amount of hydrogenated terphenyl to the non-aqueous electrolyte (see Patent Document 3).    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 9-106835    Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-058116    Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-115325